diff --git a/Cargo.lock b/Cargo.lock
index b3b64a3e6..ba05fe24b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2912,7 +2912,7 @@ dependencies = [
 [[package]]
 name = "mdbase-interop"
 version = "0.1.0-rc.2"
-source = "git+https://github.com/mdbase-dev/mdbase-spec.git?rev=5aa34fd14de2e0ae9e565425b724b306046b64ca#5aa34fd14de2e0ae9e565425b724b306046b64ca"
+source = "git+https://github.com/mdbase-dev/mdbase-spec.git?rev=f4202a5d76f189684bfb2dc34334b8e3a8017ccc#f4202a5d76f189684bfb2dc34334b8e3a8017ccc"
 dependencies = [
  "hex",
  "jsonschema",
diff --git a/Cargo.toml b/Cargo.toml
index 73d55a7ca..94916ec45 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -38,7 +38,7 @@ futures-util = "0.3"
 hkdf = "0.12"
 hmac = "0.12"
 mdbase = { path = "../mdbase-rs", default-features = false }
-mdbase-interop = { version = "0.1.0-rc.2", git = "https://github.com/mdbase-dev/mdbase-spec.git", rev = "5aa34fd14de2e0ae9e565425b724b306046b64ca" }
+mdbase-interop = { version = "0.1.0-rc.2", git = "https://github.com/mdbase-dev/mdbase-spec.git", rev = "f4202a5d76f189684bfb2dc34334b8e3a8017ccc" }
 mdbase-runtime = { path = "../mdbase-rs/crates/mdbase-runtime" }
 notify = "8"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls-webpki-roots-no-provider"] }
diff --git a/deploy/docker/Cargo.lock.hosted-provider b/deploy/docker/Cargo.lock.hosted-provider
index b3b64a3e6..ba05fe24b 100644
--- a/deploy/docker/Cargo.lock.hosted-provider
+++ b/deploy/docker/Cargo.lock.hosted-provider
@@ -2912,7 +2912,7 @@ dependencies = [
 [[package]]
 name = "mdbase-interop"
 version = "0.1.0-rc.2"
-source = "git+https://github.com/mdbase-dev/mdbase-spec.git?rev=5aa34fd14de2e0ae9e565425b724b306046b64ca#5aa34fd14de2e0ae9e565425b724b306046b64ca"
+source = "git+https://github.com/mdbase-dev/mdbase-spec.git?rev=f4202a5d76f189684bfb2dc34334b8e3a8017ccc#f4202a5d76f189684bfb2dc34334b8e3a8017ccc"
 dependencies = [
  "hex",
  "jsonschema",
diff --git a/deploy/docker/mdbase-rs-revision b/deploy/docker/mdbase-rs-revision
index 888a27a2f..dc2e827da 100644
--- a/deploy/docker/mdbase-rs-revision
+++ b/deploy/docker/mdbase-rs-revision
@@ -1 +1 @@
-76ceed008b63fd06c91dbc6ccd0cfcea043ac042
+fc1dc077a1d76677a43c047d8ad419772fc5037f
diff --git a/scripts/lib/retained-v2-upgrade.test.mjs b/scripts/lib/retained-v2-upgrade.test.mjs
index 83cf8ae25..41399e71d 100644
--- a/scripts/lib/retained-v2-upgrade.test.mjs
+++ b/scripts/lib/retained-v2-upgrade.test.mjs
@@ -43,6 +43,18 @@ test('retained-v2 is registered in the existing harness and cannot remove legacy
   }
 });
 
+test('retained-v2 freezes wire schemas, not the candidate engine revision', async () => {
+  const script = await readFile(new URL('../../test/upgrade/retained-v2.sh', import.meta.url), 'utf8');
+  assert.doesNotMatch(script, /mdbase-rs-revision/);
+  const contracts = script.match(/for contract in ([\s\S]*?); do/);
+  assert.ok(contracts);
+  for (const schema of ['application-capability-catalog.v1.json', 'application-capability-catalog.v2.json', 'operation-catalog.v1.json']) {
+    assert.ok(contracts[1].includes(`packages/protocol/schemas/${schema}`));
+  }
+  assert.match(script, /cmp <\(git .*PREVIOUS_RELEASE_COMMIT:\$contract/);
+  assert.match(script, /diff --exit-code .* -- crates\/connect-hosted-provider\/migrations/);
+});
+
 test('retained-v2 rejects a mutable beta94 tag before starting resources', async () => {
   const root = new URL('../../', import.meta.url).pathname;
   // An image tag is rejected before even consulting Git or Docker.
diff --git a/test/upgrade/retained-v2.sh b/test/upgrade/retained-v2.sh
index 91c87082f..6dbdd1df3 100644
--- a/test/upgrade/retained-v2.sh
+++ b/test/upgrade/retained-v2.sh
@@ -21,10 +21,10 @@ retained_v2_inputs() {
     jq -e '.phase == "compatibility-prelude" and .fresh_semantic_versions == [1]' >/dev/null
   jq -e '.phase == "v2-enablement" and .fresh_semantic_versions == [1,2]' \
     "$repo_root/config/application-issuance-policy.json" >/dev/null
-  # Retained reader/engine and schema bytes are immutable across this pair.
+  # Keep wire schemas immutable; engine revisions may advance. The scenario
+  # below tests compatibility by running the candidate and exact predecessor.
   local contract
-  for contract in deploy/docker/mdbase-rs-revision \
-      packages/protocol/schemas/application-capability-catalog.v1.json \
+  for contract in packages/protocol/schemas/application-capability-catalog.v1.json \
       packages/protocol/schemas/application-capability-catalog.v2.json \
       packages/protocol/schemas/operation-catalog.v1.json; do
     cmp <(git -C "$repo_root" show "$MDBASE_CONNECT_PREVIOUS_RELEASE_COMMIT:$contract") "$repo_root/$contract"